Anyone can flag a post or comment (look for that "flag" on the top line), and it does not result in any notification.
No, I have an old post that's marked as "flagged" but not that one.
Posts don't get marked as "flagged" or "dead" on the first "flag" by someone: I ain't sure of the threshold, and I am merely guessing, but trying to tell you not to bother with this — it doesn't really matter, HN has some non-transparent rules and we all accept to live by them believing they are right in spirit most of the time.